This invention relates to electrically operated control systems for controlling operation of gas fired, induced draft heating apparatus in which a main burner is directly ignited by a hot surface igniter.
Due to the ever present need to conserve energy, many improvements have been made in recent years in the construction and operation of gas fired furnaces utilized in residential dwellings. A particularly popular construction is of a type which utilizes direct ignition of the main burner by a hot surface igniter whereby the conventional standing pilot is omitted. Some of such direct ignition furnaces further include the feature of induced draft wherein an inducer pulls in the air required for combustion and forces out, through the flue, the products of combustion.
In some of such direct ignition, induced draft furnaces, the combustion chamber and/or the flue are so constructed as to enable the furnace to operate at a relatively high rate of efficiency. For example, the combustion chamber and/or flue in such furnaces often includes tortuous paths effective for enabling highly efficient transfer of the heat generated by the burner flame to usable heated air for heating the dwelling. Because of such combustion chamber and/or flue construction, the quantity of fluid flow which can be effected by the inducer is inherently limited. While limited, the quantity of fluid flow must be adequate to provide safe operation of the system. The prior art discloses a variety of control systems for controlling operation of such high efficiency furnaces. Examples of such systems are shown in U.S. Pat. Nos. 4,925,386 and 5,022,460.
In some direct ignition, induced draft furnaces, the combustion chamber and/or flue are not as restrictive to fluid flow effected by the inducer as the high efficiency furnaces described in the preceding paragraph. While such furnaces do not provide the same high efficiency, they provide an efficiency higher than those furnaces utilizing the conventional standing pilot. Also, they are considerably less expensive than the high efficiency furnaces. While the control systems referenced in the preceding paragraph can be utilized in such less efficient furnaces, such control systems are relatively expensive to manufacture and provide features, such as precise time periods, not required in the less efficient furnaces. For example, in such less efficient furnaces, the quantity of fluid flow effected by the inducer is not as limited as in the high efficiency furnaces. Therefor, the less efficient furnaces can safely tolerate longer and less precise time periods wherein gas may be flowing and no flame exists. It would be desirable to provide a control system that would provide the control functions required in such less efficient, direct ignition, induced draft furnaces at a considerably less cost to manufacture than the prior art systems.